The atmosphere is host to an omnipresent bacterial community that may influence fundamental atmospheric processes such as cloud formation and precipitation onset. Knowledge of this bacterial community is scarce, particularly in air masses relevant to cloud formation. Using a light aircraft, we sampled above the atmospheric boundary layer-that is, at heights at which cloud condensation occurs-over coastal areas of Sweden and Denmark in summer 2009. Enumeration indicated total bacterial numbers of 4 x 10(1) to 1.8 x 10(3) m(-3) air and colony-forming units of 0-6 bacteria m(-3) air. 16S rRNA gene libraries constructed from samples collected above the Baltic Sea coast revealed a highly diverse bacterial community dominated by species belonging to the genera Sphingomonas and Pseudomonas. Bacterial species known to carry ice-nucleating proteins were found in several samples. Modeled back trajectories suggested the potential sources of the sampled bacteria to be diverse geographic regions, including both marine and terrestrial environments in the northern hemisphere. Several samples contained 16S rRNA genes from plant chloroplasts, confirming a terrestrial contribution to these samples. Interestingly, the airborne bacterial community displayed an apparent seasonal succession that we tentatively ascribe to in situ succession in the atmosphere.